School of Chemistry - Theses

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    Synthesis, structure and reactivity of ligand stabilized coinage metal nanoclusters
    Zavras, Athanasios ( 2013)
    The coinage metal nanoclusters (CMNCs), defined as copper, silver or gold, constitute an intermediate state of matter that exist between molecules and bulk material. The properties of CMNCs differs to that of molecules and bulk material due to quantum confinement effects. These nanostructured materials have attracted significant attention owing to their fundamentally interesting architectures, and unique properties with applications in areas such as catalysis, optical materials, medical imaging, models for hydrogen storage. Tailoring the properties of such promising materials has proven challenging and requires a fundamental understanding of their assembly, structure and reactivity. The aim of this thesis is: (i) the primary application of mass spectrometric techniques to monitor the formation of CMNCs which result from the addition of sodium borohydride to a solution consisting of a coinage metal salt and the bidentate ligand, bis(diphenylphsphino)methane (dppm) under various synthetic conditions; (ii) to apply this information in developing synthetic approaches to optimize clusters of interest and apply a mass spectrometry (MS) directed synthesis leading to the isolation of crystalline material suitable for structural characterization by X-ray crystallography (iii) apply MS based analysis methods to provide information on the reactivity of CMNCs in solution and the reactivity and structure of mass selected CMNCs in the gas phase. Electrospray ionization mass spectrometry (ESI-MS) and UV-Vis spectroscopy were used to monitor the formation of gold nanocluster cations in the condensed phase via the sodium borohydride (NaBH4) reduction of methanolic solutions containing AuClPPh3 and dppm. ESI-MS highlights the formation of complexes prior to the addition of NaBH4 as [Au2(dppm)2]2+, [Au(PPh3)2]+, [Au2(dppm)3]2+, [Au(dppm)2]+,[Au2Cl(dppm)2]+. The cationic complex product distribution can be monitored over a range of metal to ligand ratios to minimize the colloid precursor [Au(PPh3)2]+. The addition of NaBH4 where the optimized metal to ligand ratio was determined as AuClPPh3:dppm is 1:2 results in the formation of the following types of gold nanoclusters [Au9(dppm)4]3+, [Au9(dppm)5]3+, [Au5(dppm)3(dppm-H+)]2+, [Au10(dppm)4]2+, [Au11(dppm)5]3+, [Au11(dppm)6]3+, [Au13(dppm)6]3+ and [Au14(dppm)6(Ph2PCHPPh2)]3+. The gas phase unimolecular chemistry of these cations was examined by (i) collision induced dissociation (CID) and electron capture dissociation resulting in the gas phase synthesis of the novel clusters [Aux(dppm)y]z+ (x = 2,3 , 6–13; y = 1–6 and z = 1–3) and [Aux(dppm)y(dppm-H+)]z+ (x = 5,14; y= 2,5; z = 2,3) via ligand loss and core fission fragmentation channels. (ii) electron capture dissociation (ECD) of mass selected multiply charged gold cluster cations where an additional fragmentation channel arises due to C-P bond activation. ESI-MS was also applied to study the reactivity that results from silver salts in the presence of dppm, that are treated with sodium borohydride. It was observed by ESI-MS that no all metallic silver clusters had formed. Instead there existed abundant and relatively monodisperse trinuclear silver(I) hydride clusters. The synthesis could be refined by careful MS based analysis to result in the isolation of crystalline material of (i) [Ag3(μ3-H)(μ3-Cl)(dppm)3]BF4, and (ii) [Ag3(μ3-H)(dppm)3](BF4)2. These clusters could be mass selected to generate novel gas phase clusters in the gas phase. The multiply charged cation [Ag3(μ3-H)(dppm)3]2+ was also investigated by ECD and EID. The silver hydride cluster cation [Ag10H8(dppm)6]2+ was observed during the synthesis of trinuclear silver clusters. This cluster has yet to be isolated.